The present invention relates to self-contained imaging sheets for use in copy systems such as pressure-sensitive carbonless paper, and, more particularly, to dual layer self-contained imaging sheets incorporating a hollow microspherical plastic pigment.
Carbonless copy systems and other copying systems employing a color precursor and a developer are well known in the art. These systems can comprise two sheets, a transfer or donor sheet which contains a colorless color-forming agent on one of its surfaces which is often contained in pressure rupturable microcapsules and a developer or receiver sheet, which is typically a substrate having a layer of a developer material coated onto its front surface which can react with the colorless color-forming agent to form a visible image. In practice, to produce an image, the two sheets are juxtaposed so that the colorless color-forming agent faces the developer material. Upon the application of pressure, such as from a pen, pencil, typewriter, or other writing instrument, the microcapsules are ruptured which releases the colorless color-forming agent. The color-forming agent reacts with the developer material on the developer sheet to form a visible image.
Transfer pressure-sensitive systems as described above are only one type of imaging system employing a color precursor and a developer. Other types that are known in the art are self-contained systems in which the color precursor and the developer are carried together on the same support (e.g., in the same or separate layers). Another type of copy system which employs a color precursor and a developer is thermal recording systems in which the color precursor is activated through the application of heat. Light sensitive copy systems such as the CYCOLOR system described in U.S. Pat. No. 4,399,209 are still another type of system which relies upon the interaction of a color precursor and a developer to form images.
Some conventional self-contained carbonless papers utilize a two layer coating system whereby a microcapsule-containing CB coating is applied to a substrate and then overcoated with a developer-containing CF coating. The CF coating functions not only as a developer, but also as a protective overlayer to prevent premature rupture of the pressure sensitive microcapsules in the CB coating. The dual layer self-contained structure provides improved smudge resistance and minimizes capsule damage during routine handling. One drawback associated with conventional two layer CF/CB self-contained coating systems is that a relatively high coat weight, typically around 2.9 #/ream (1300 ft2), must be applied to produce a self-contained paper having suitable imaging characteristics. Combined coating containing both developer and microencapsulated color precursor can be applied at lower coat weights, but can result in unacceptable degrees of smudge and/or discoloration without a protective overcoat.
The present invention provides a self-contained sheet and self-contained systems including the same which provides improved image intensity and resistance to smudge. The self-contained sheet is characterized in that it utilizes a dual layer coating construction to improve the appearance of the sheet and the imaging performance of the self-contained system. The self-contained imaging sheet of the invention comprises: a support; a first layer of microcapsules containing a color precursor of the electron donating type in a carrier liquid on the surface of the support; a layer of hollow microspheres overlying the microcapsule layer; and a developer material present in at least one of the microcapsule layer or microsphere layer wherein the developer material can react with the color precursor to form a visible image when the color precursor is released from the microcapsules.
The dual layer structure of the self-contained imaging sheet is believed to be advantageous because the microspheres overlying the microcapsule layer appear to provide an opaque, energy absorbing protective barrier over the microcapsules. Furthermore, upon the application of an image-forming pressure, the microspheres contained in the top coat layer are believed to locally rupture, collapse, or dissolve and thereby form an interconnected network of microscopic voids. The microscopic voids are believed to provide improved image intensity as a result of increased surface area of the developer materials and capillary action. When a visible image is produced by the reaction of a colorless chromogenic material with the particulate developer, it is desirable to have a large number of pores in the layer containing the developer. According to one capillary model for oil transfer, the density of the image produced is proportional to the square root of the effective capillary radius of the developer material. Accordingly, an increase of the number of pores and/or effective capillary radius pore size increases the ease with which the color precursor contacts and reacts with the developer, thereby increasing the image density and speed of image development. The plastic pigment microspheres further enhance the image by contributing additional whiteness and opacity to the self-contained sheet.
One embodiment of the present invention is a self-contained sheet which provides faster development and improved image intensity through one or a combination of mechanisms. The self-contained sheet comprises a support having a microcapsule layer on one side thereof, a layer of hollow microspheres overlying the microcapsule layer and a developer material in at least one of the microcapsule layer or microsphere layer wherein the developer material can react with the color precursor to form a visible image when the color precursor is released from the microcapsules.
In one embodiment the present invention provides for a self-contained paper utilizing microspheres applied as a separate top coating over a reactive imaging layer of microcapsules and developer material. This construction of the self-contained paper is advantageous in that it provides the performance associated with a single layer self-contained coating such as image speed and density while minimizing potential drawbacks of the single layer self-contained coating such as premature capsule damage from handling (smudge) and discoloration and yellowing of the developer because the microsphere overlayer functions as a protective coating.
The present invention also provides for a self-contained paper utilizing a layer of microspheres and developer applied as a top coating over a layer of microcapsules. This construction of the self-contained paper is advantageous because the top coating again protects the microcapsules from premature rupture and creates a matrix of developer particles surrounding the microcapsules. Speed of image development and image intensity are enhanced by the proximity of the microcapsules to the developer particles. Upon the application of an imaging force, the encapsulated chromogenic contents are released from the microcapsules and react more quickly with the developer particles surrounding the microcapsules to form a visible image. Preferably, the microspheres and developer layer further comprises a high concentration a of mineral pigments which are believed to facilitate image formation by providing faster and more complete reaction of the chromogenic materials and the developer. Mineral pigments are believed to disperse developer particles more completely, thereby reducing agglomerates of developer particles. Color is developed more effectively by individual, separate particles.
Other objects and advantages of the present invention will be more fully understood and appreciated by reference to the following description and the appended claims.